An ECG (Electrocardiogram) waveform is obtained by observing and recording the electrical activity of a heart, and is measured by attaching electrodes to a body surface in a general method. As an ECG waveform lead system, that is, an electrode arrangement, there are various types using limbs and chest. In the V3 to V5 leads among precordial leads, an electrode is arranged at the left chest. In the CC5 lead suitable for monitoring an ECG waveform for a long time, electrodes are arranged at the symmetrical positions of left and right chests. These leads have the advantage that a stable waveform having a large amplitude is obtained.
FIG. 9 shows an example of an ECG waveform. In FIG. 9, the ordinate represents the electric potential and the abscissa represents the time. The ECG waveform is formed from continuous heartbeat waveforms, and one heartbeat waveform is formed from components such as P, Q, R, S, and T waves reflecting the activities of atriums and ventricles.
It is known that biological information such as an R−R interval obtained from an ECG waveform is an index reflecting the autonomic activity. It is useful for evaluation of the autonomic function to obtain an ECG waveform in daily life and analyze data of a heartbeat fluctuation from detected heartbeats. Furthermore, there is an application in which an exercise tolerance is estimated from heartbeat data during an exercise, and used for optimization or the like.
As a conventional heartbeat detection method, the following literatures are known. Japanese Patent Laid-Open No. 2002-78695 discloses an arrangement for removing the fluctuation of the baseline of an ECG waveform. In addition, Japanese Patent Laid-Open No. 2003-561 discloses an arrangement of recognizing an R wave using a threshold based on an amplitude between the peak and valley of a waveform.
A method of obtaining the R−R interval or the like based on a change in value obtained by calculating the first derivative of an ECG waveform is described in “ECG Implementation on the TMS320C5515 DSP Medical Development Kit (MDK) with the ADS1298 ECG-FE”, Texas Instruments Incorporated, <http://www.ti.com/lit/an/sprabjl/sprabjl.pdf>, 2011. In this heartbeat detection method, more specifically, the absolute value of the difference between the (n+1)th sampling value and the (n−1)th sampling value is obtained, peaks are detected based on a threshold, and then the time width between two peaks is set as the R−R interval.